Nonmuscle myosin IIs, together with actin, form the basic contractile machinery involved in a number of cellular processes such as cytokinesis and cell migration. Three isoforms of nonmuscle myosin heavy chain II (NMHC) have been identified in vertebrates, namely, NMHC II-A, II-B and II-C. They have similar molecular structures: each heavy chain contains a globular region at the amino terminus that catalyzes ATP hydrolysis and binds to actin to generate force, and an alpha-helical carboxyl-terminal coiled-coil tail region that is responsible for myosin filament assembly. These isoforms are also well conserved with a 64-80% identity in amino acid levels throughout the whole molecule. The expression of all three myosins seems to be regulated in a tissue-specific and developmentally-controlled manner. Previous work showed that ablation of NMHC II-A results in defects in cell adhesion and visceral endoderm maturation, and is lethal by embryonic day (E) 7.5. Ablation or mutation of NMHC II-B results in specific defects in the heart and brain and is lethal by E13.5-16.5. To further understand the functions of NMHC II-A and II-B and in an effort to avoid embryonic lethality, we used homologous recombination to ablate the message encoded by Myh-9 by inserting cDNA encoding human mCherry-NMHC II-A or GFP-NMHC II-B or GFP-NMHC II-AB (II-A head and II-B tail) or GFP-NMHC II-BA (II-B head and II-A tail) cDNA into the first coding exon at the mouse II-B locus. This ablated NMHC II-B and placed the expression cassette under control of the endogenous II-B promoter. Interestingly, we have obtained a very high ES cell recombinant efficiency (67-96%). Knock-out and knock-in mice are currently being generated along with homozygous ablated and mutated ES cells.